The effects of an applied magnetic field on the electronic and far-infrared properties of impurity states in donor-doped GaAs-Ga1-xAlxAs quantum wells are studied within a variational scheme in the effective-mass approximation, with the magnetic field applied perpendicular to the quantum-well interfaces. Theoretical calculations for the Is-like hydrogenic ground state, and first excited 2p(+/-)-like states are presented for different values of the magnetic field, and as functions of the quantum-well thickness and position of the donor within the well. The absorption spectra related to 1s-2p(+/-) intradonor transitions for x-polarised radiation are calculated and results are in quite good agreement with previously reported magneto-spectroscopic data.983215219